Accelerometers equipping flying machines are used to work out their attitude parameters. However, when these machines are, in particular, helicopters, these accelerometers are subjected to strong accelerations generated by the vibrations of these helicopters, which are superposed on the actual acceleration of the bearer. A typical specification of vibrations has, for example, a white noise level of 0.02 g2/Hz between 10 and 300 Hz to which sine lines are added with an amplitude up to 2.5 g at low frequency.
Furthermore, a new generation of accelerometric sensors, based on MEMS (Micro Electro Mechanical Systems) technology is currently coming to light. These sensors, initially intended mainly for motor vehicles, have performance compatible with low cost attitude systems for aircraft, such as the systems called “Attitude and Heading Reference Systems” (AHRS) or standby horizon systems. However, their measurement range is generally limited (typically 1 to 3 g).
The use of such accelerometers in the case of helicopters poses two problems:
1. Saturation. In the case of accelerometers such as, for example, the VTISCA 61T, the nominal performance range is located at 2 g and the saturation threshold is found at 2.7 g. The bandwidth of the sensor is less than 80 Hz.
The effective value of white noise seen through the transfer function of the accelerometer is 1.3 g. By assimilating the peak value to the value at 3σ, the white noise itself may generate 3.9 g at peak. Added to this white noise is one of the previously mentioned sine lines, which are all in the bandwidth of the accelerometer. The risk of saturation is therefore very high.
It is difficult to evaluate the impact of the saturation of accelerometers on the attitude performance, as this depends on the symmetry of the saturation. It is, however, not acceptable to have saturation in nominal operation.
2. Rectification. Typical rectification coefficients of accelerometers of the type envisaged are 5 mg/g2. With an effective value of 1.3 g, a bias of around 8.5 mg is generated. When account is taken of the sine lines, the overall attitude error obtained is greater than 2°. As the attitude specification of an AHRS is typically 1° (at 95%), these values are not acceptable.
The outputs from the AHRS may also be used for piloting the aircraft. In this case, a typical specification value is 10 mg (95%). This specification also cannot be met.
Among the various solutions conceivable for solving these problems, it is possible to mention:                2. Using an accelerometer having both a range sufficient not to saturate and a low rectification coefficient. This is the solution currently adopted in AHRSs. Accelerometers having both a sufficiently high range to avoid saturation and a low rectification coefficient are of the controlled pendulum or VBA (Vibrating Beam Accelerometer) type. (The Honeywell QA 700 and SAGEM ACSIL models may be cited). Their cost is prohibitive for the intended applications.        2. A dual accelerometer solution: an accelerometer of range 2 g allowing conservation of the repeatability of long term bias associated with a large range accelerometer (typically 10 or 12 g). Thus the problem of saturation is solved, but not that of rectification. Indeed, low cost 12 g accelerometers have a much higher bandwidth (500 Hz) and a rectification coefficient that is also higher.        3. Suspended sensors solution. The first objectionable line is typically around 20 Hz. It would therefore be necessary to cut off the frequency spectrum below this value, which is difficult to conceive as very large amplitudes of displacement and problems of stability of misalignment are then encountered. It is also not possible to locate the suspension frequency between two lines as they are very close together.        